1. Technical Field of the Invention
This invention relates to the purification of propylene oxide. More particularly, this invention relates to a distillation process for removing contaminating quantities of impurities including oxygen-containing impurities such as methanol, acetone and water from an impure propylene oxide feedstock. Still more particularly, this invention relates to a method wherein an impure propylene oxide feedstock contaminated with from about 50 to about 4,000 ppm of methanol, from about 0.01 to about 2 wt. % of water and from about 0.01 to about 2 wt. % of acetone is purified in an extractive distillation column using as the extractive distillation agent, a mixture of triethylene glycol with a lower boiling solvent selected from the group consisting of 2-methyl-2,4-pentanediol, tertiary butyl alcohol and ethylene glycol.
2. Prior Art
In copending Marquis et al. U.S. patent application Ser. No. 07/770,493, filed Oct. 3, 1991 and entitled "Purification of Propylene Oxide by Extractive Distillation using Triethylene Glycol" (D#81,112), there is disclosed, as indicated by the title, a method for the purification of propylene oxide wherein triethylene glycol is used as the extractive distillation agent.
It is known to react a hydroperoxide feedstock such as tertiary butyl hydroperoxide with propylene in the presence of an epoxidation catalyst in order to provide a reaction product comprising propylene oxide, an alcohol corresponding to the hydroperoxide feedstock, a solvent, and impurities (see, for example, Kollar U.S. Pat. No. 3,350,422, Kollar U.S. Pat. No. 3,351,635 and Sorgenti U.S. Pat. No. 3,666,777.
It is also known to separate the reaction product by distillation in order to obtain a plurality of fractions including, for example, a propylene recycle fraction, a propylene oxide product fraction, an alcohol fraction, etc.
It is also known that methanol, acetone and water are common contaminants for propylene oxide which are removed only with difficulty.
For example, Mitchell et al. U.S. Pat. No. 2,550,847 is directed to a process for separating purified propylene oxide from a crude propylene oxide product contaminated with acetaldehyde, methyl formate, methanol, etc., by treating the crude mixture with an aqueous basic substance followed by recovery of the purified propylene oxide by any suitable means such as by decantation. Mitchell et al. reported a recovery of a product containing 78 to 82 wt. % of propylene oxide which, they stated, could be increased in purity to about 95 to 99% by fractional distillation.
Robeson et al. U.S. Pat. No. 2,622,060 discloses a process for the purification of propylene oxide contaminated with impurities, including methanol, by subjecting the impure propylene oxide to distillation in the presence of an extractive distillation agent comprising an aqueous solution of an alkali. The inventors report in Example 1 of their patent a method wherein 500 parts by weight of a crude propylene oxide fraction was extractively distilled in accordance with their invention to obtain 325 parts by weight of a product containing about 99.6 wt. % of propylene oxide. Thus, a significant loss of propylene oxide occurred during the process.
In a process unrelated to the purification of propylene oxide, Goddin et al. in U.S. Pat. No. 2,751,337 disclose a process for separating acetone from a mixture of acetone with methanol and methyl acetate utilizing water as an extractive distillation agent.
Hamlin et al. in U.S. Pat. No. 3,409,513 disclose the hydro-extractive distillation of mixtures comprising acetone, lower aliphatic alcohols and esters of lower aliphatic alcohols with carboxylic acids. It is pointed out by the patentees that acetone, methyl acetate and methanol form an azeotrope boiling at 55.5.degree.-56.5.degree. C. Hamlin et al. propose to recover partially purified acetone from such a ternary azeotrope by liquid-liquid extraction with water followed by hydro-extractive distillation of the aqueous phase in order to obtain a partially purified acetone fraction.
Washall U.S. Pat. No. 3,578,568 discloses a process for removing oxygen-containing impurities such as acetone, acetaldehyde and methanol from impure propylene oxide using a glycol such as ethylene glycol or propylene glycol as an extractive distillation agent.
Hoorl and Newman U.S. Pat. No. 3,632,482 is directed to a propylene oxide recovery process by extractive distillation using an alcohol-ketone-hydrocarbon solvent. The invention relates to a method for the purification of crude propylene oxide contained in a mixture produced by the epoxidation of propylene with an organic hydroperoxide and calls for extractive distillation of the crude propylene oxide in a plurality of successive extractive distillation zones with the aid of a solvent mixture consisting essentially of hydrocarbons, alcohols, and/or ketones corresponding to the organic hydroperoxide employed in producing the propylene oxide. In the preferred embodiment of their invention, the extractive distillation agent is a recycle fraction from a three column distillation sequence wherein the bottoms from the third distillation column are flashed to obtain an overhead composed of hydrocarbons, alcohols and/or ketones which is recycled as an extractive distillation agent to the three distillation columns involved in the propylene oxide purification sequence.
Burns et al. U.S. Pat. No. 3,715,284 discloses a process for the purification of impure propylene oxide using acetone or a mixture of acetone with methanol which is introduced into a distillation column either below or together with the impure propylene oxide.
Schmidt U.S. Pat. No. 3,881,996 is directed to a distillation sequence employing at least three and optionally four columns for the purification of crude propylene oxide, one of the columns optionally being an extractive distillation column wherein a hydrocarbon such as octane is used as the extractive distillation agent.
Schmidt U.S. Pat. No. 4,140,588 discloses a process for the purification of propylene oxide contaminated with methanol and acetone using water as an extractive distillation agent, the water being introduced into the distillation column above the point of introduction of the crude propylene oxide feed.
Schmidt states at column 2, lines 50-55 that: "Propylene oxide, however, has a substantial solubility in water and is readily hydrolyzed to propylene glycol (PG) in the presence of large amounts of water"--i.e., in the reboiler section of the tower.
U.S. Pat. No. 4,971,661 discloses the use of an aqueous acetone extraction to remove methanol from propylene oxide.
U.S. Pat. No. 3,578,568 discloses the use of glycols or glycol ethers in an extractive distillation to remove oxygen containing impurities such as acetone, acetaldehyde, and methanol. It is claimed that the concentration of the solvent in the vapor space in the extractive distillation zone of the distillation tower is preferably between 15 and 50 mole percent of the total vapor.
Compared to U.S. Pat. No. 3,578,568, this invention uses considerably lower concentrations of solvent in the extractive distillation zone to remove water and oxygen-containing impurities such as acetone. Since the concentration of the triethylene glycol is lower, the size and heat requirements of the associated triethylene glycol regenerator are reduced.
U.S. Pat. No. 4,140,588 discloses the use of water in an extractive distillation to remove methanol and acetone.
U.S. Pat. No. 3,607,669 discloses the use of a C.sub.8 to C.sub.12 hydrocarbon to separate propylene oxide from water.
Shih et al. U.S. Pat. No. 5,000,825 discloses the purification of monoepoxides such as propylene oxide that are contaminated with oxygenated impurities such as water, low molecular weight alcohols, low molecular weight ketones, low molecular weigh aldehydes and the like by the extractive distillation of the contaminated monoepoxide using a lower glycol containing 2 to 4 carbon atoms. Examples of lower glycols that are given in the patent include ethylene glycol, 1,2-propane diol, 1,3-propane diol, 1,4-butane diol, 1,2-butane diol, 1,3-butane diol and 2,3-butane diol. It is stated that higher diols or higher glycol ethers do not provide sufficient selectivity for the removal of such impurities and are not included as the extractive distillation solvents suitable for use in the invention.